Generally, fatty foodstuffs such as raw meat, processed meat and cheese are irregular in shape and are more often than not sold in a boned form or in a form accompanied by various accessorial articles (primary packaging materials such as, for example, plate and net).
In the packaging of these fatty foodstuffs of irregular shape, there has heretofore been used the so-called shrink-packaging method which includes wrapping the foodstuffs with shrinkable films. This method proves most convenient for the purpose. In this respect, vinylidene chloride copolymer resins (hereinafter referred to as "VDC resins" for short) excel in gas-impermeability, oil resistance and sealing with wire clips as well as in shinkability and, for this reason, find widespread acceptance for use in shrinkable films.
The VDC resins which are in use today contain from 6 to 10% by weight of additives such as plasticizers and stabilizers which have been incorporated therein for the purpose of imparting the flexibility necessary for enhancing cold resistance and the ability to seal with clips and further for the purpose of conferring an advantageous film-forming property. Use of VDC resins, therefore, can at times result in a hygienically undesirable situation where the additives present in the resins migrate into some, if not all, foodstuffs which are exposed to direct contact with the resin films. It may also happen that the resins, if used on heavy foodstuffs, fail to provide sufficient physical strength, especially cold resistance property.
Various attempts have heretofore been made to cure the VDC resins of such disadvantages. For example, there has been suggested a three-layer laminate film which is produced by interposing a layer of a VDC resin between a pair of outer layers each made of an ethylene-vinyl acetate copolymer resin (hereinafter referred to as "Et-VA resin" for short) excelling in cold resistance property and adhesiveness with VDC resins and subsequently causing the three layers to be simultaneously coextruded as a single construction (as disclosed in Canadian Pat. No. 982,923, for example). There has also been suggested a three-layer laminate film of a similar construction differing in the cross-linking of one of the outer Et-VA resin layers (one of the two outer layers of the laminate film) by exposure to radiant rays (as disclosed in Japanese Patent Laid-Open Publication No. 34565/1972, for example). These conventional multi-layer laminate films invariably have at least one of the outer layers made of an Et-VA resin which is notably deficient in oil resistance property (namely, the outer layer of Et-VA resin is not cross-linked by exposure to radiant rays). In the packaging of fatty foodstuffs by use of these films, if the films are used in such a manner that their non-crosslinked outer layers fall on the outermost side, then such outer layers are inevitably susceptible to adhesion of fats or oils in the course of the packaging process. The innermost layers of these laminate films which are exposed to direct contact with foodstuffs, therefore, are required to possess an oil resisting property where the foodstuffs are of a fatty nature. This means that in those multi-layer laminate films which are used for packaging fatty foodstuffs, the outermost layers and innermost layers are both required to passes an oil resisting property. The fatty foodstuffs thus wrapped in such multi-layer laminate films are dipped in a hot water bath kept about 95.degree. C. so as to shrink the laminate films. During this shrinking process, the outermost layers of the laminate films are severely tested with respect to their oil resistance. The outermost layers are, accordingly, required to have sufficient resistance to both heat and oil at such elevated temperatures. In this respect, the multi-layer laminate films produced by the conventional techniques described above are not completely satisfactory.
Prior to the shrinking process, the fatty foodstuffs of irregular shape are wrapped in multi-layer laminate films, evacuated to a vacuum and clipped at opposite open ends with wire clips. As the result, a vacuum occurs and remains between the films and the foodstuffs where the packaged articles contain voids. During the shrinking process, portions of the films adjacent voids are exposed to the stress of thermal shrinkage and to the external pressure due to the presence of a vacuum and, furthermore, are softened by heat and oil. Such affected portions of the films are inevitably stretched to an excessively small thickness, with the result that the permeability and strength are degraded possibly to the extent of causing the most serious problem of rupture. What is more, the multi-layer laminate films produced by the conventional techniques invariably have their seams heat sealed and, therefore, have not been completely freed from the problem that their sealed layers, upon exposure to an intense heat of 95.degree. C. and in the presence of an oil, are swelled with the oil and are consequently degraded in strength, occasionally to the extent of sustaining rupture along the seams.